In the prior art, various methods have been utilized to monitor the progress of a flood front in oil recovery operations. The first is to track the amount of oil and water recovered in production wells and to compare that to the quantity of water being injected into the system. Then computer models are created which include known information about the formation being flooded. The disadvantage of only monitoring the flow rates is that if the formation is not homogeneous then valuable pockets of hydrocarbon might not be recovered.
The other method is disclosed in U.S. Pat. No. 3,874,451. It provides for the detection of the arrival of the flood front by monitoring the pressure change in boreholes. This method requires that the boreholes used for pressure monitoring must be uncased. In a production reservoir this can require the removal of casing already present in the boreholes or the drilling of new, uncased boreholes.
Then, U.S. Pat. No. 4,085,798, discloses a method for monitoring the flood front profile during water flooding by adding a tracer element having a characteristic gamma ray emission energy to the flood fluid. It is recognized as a serious disadvantage to be required to add tracer elements to the flood fluid prior to injection. Since this method is only directed to detecting elements in the injection fluid it does not provide an indication of flood front movement until the fluid flood front reaches or nearly reaches the monitor boreholes.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing an environmentally friendly high resolution method for monitoring the flood front movement.